Gusset Expansion Envelopes | C3 C4 C5 DL Peel Seal

When the contents are too thick for a standard envelope, a gusset expansion envelope provides the space without the need to move to a box. The expanding sides of a gusset envelope accommodate bulky documents, multi-page contracts, reports, catalogues, and legal bundles without straining the seams. Postal Packaging's range covers sizes from C6 through to C3, spanning 162x114mm up to 457x324mm, with peel and seal closure as standard on most lines. The gusset typically expands to 25mm, enough for a substantial document pack or several brochures. Available in white and manila finishes to suit both formal correspondence and practical dispatch.

6 products

Manila String & Washer Gusset Envelopes

From £0.71 ~~£0.79~~

Black String and Washer Gusset Envelopes

From £0.76 ~~£0.84~~

White String & Washer Gusset Envelopes

From £0.84 ~~£0.93~~

Eco Product

Recycled Manila Gusset Peel & Seal Envelopes

From £0.25

Red String & Washer Gusset Envelopes

From £0.76 ~~£0.84~~

Eco Product

Recycled White Gusset Peel & Seal Envelopes

From £0.23

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More About "Gusset Expansion Envelopes"

The Practical Solution for Thick Documents

A standard envelope is designed for a single sheet or a small number of pages. When contents exceed that, the envelope fails: the flap will not seal, the seams split, or the document folds under pressure. A gusset envelope solves this directly. The expanding base and sides create depth that a flat envelope cannot, and the peel and seal strip closes cleanly regardless of how much is inside.

When to Use a Gusset Envelope

Multi-page legal or financial documents that cannot be folded
Reports and proposals sent flat in C4 or C3 format
Catalogues, brochures, and promotional packs
Medical records or case files requiring a secure, flat envelope
Multiple documents dispatched together as a set

Size Guide

C6 (162x114mm): suitable for A6 documents folded twice, or small multi-page packs
DL (220x110mm): standard long document format with added depth for thick letters
C5 (229x162mm): A5 documents flat, or A4 folded once with several sheets
C4 (324x229mm): A4 flat, the most common size for reports and document packs
C3 (457x324mm): A3 flat, or large-format document bundles

Gusset Depth and Closure

Standard gusset depth is 25mm, which typically accommodates 20 to 40 sheets at 80gsm. Peel and seal closure is used on most lines for fast, secure sealing without moisture. String and washer gusset formats are also available for documents that require repeated access.

Customer Feedback

What Our Customers Say

See why we’re trusted as by thousands of UK businesses for packaging and envelopes through our customer reviews.

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Envelope Printing Customer

Branded envelopes that felt event-ready

We needed branded envelopes for a client event and they looked exactly right. The team were helpful from start to finish and made sure everything moved quickly so we could meet our deadline. A really smooth experience for printed envelopes.

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Envelope Customer

Quick fix after an order issue

My order arrived with a problem, but after I emailed the team I received a quick response and the issue was sorted straight away. It was handled professionally and made me feel confident about ordering again.

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First Time Customer

Fast support when delivery went wrong

There was an issue with the courier delivering to my address, but the team dealt with it quickly and kept things moving. A replacement was arranged and the previous order was refunded without any fuss. It gave me a good first impression and I would order again.

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Wholesale+ Member

Reliable support for regular orders

The courier caused a problem, but Jess and Kelly were quick to help and did everything they could to sort it. They also looked at the products I order regularly and helped make future orders easier. Good value, good quality and service that feels genuinely useful.

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Envelope Printing Customer

Easy custom printed envelope order

Ordering custom printed envelopes was straightforward from the start. James answered my questions clearly, the order arrived when expected and the finished envelopes were great quality. I would be happy to use the service again.

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Wholesale+ Member

Helpful and easy to deal with

A very straightforward company to buy from. The team were helpful, clear with communication and easy to deal with, which makes a big difference when ordering packaging regularly.

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Sample Customer

Helpful advice before ordering

The team helped point me towards the right product before I placed my order. It made the decision much easier and gave me confidence that I was choosing the right packaging for what I needed.

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Packaging Customer

Useful support for flat pack cartons

Collection: Gusset Expansion Envelopes